With oil reserves being depleted, the possibility of using fuel cells as an alternative means to provide electrical energy is attracting ever-increasing interest. Of the many types of fuel cell devised to date, proton exchange membrane fuel cells (PEMFCs) are of greater and greater potential with the world moving towards a hydrogen-based technology. PEMFCs can cleanly and efficiently convert the chemical energy of hydrogen and oxygen into water and electrical & thermal energy.
In PEMFCs, hydrogen and oxygen react at separate electrodes—anode and cathode respectively—with the hydrogen being disassociated at the anode with the use of a catalyst into protons and electrons. The protons so generated diffuse through the electrically insulating polymer electrolyte membrane and the electrons travel by an external load circuit to the cathode, the passage of the electrons along this external load circuit providing the current output of the fuel cell. At the cathode, molecular oxygen combines with the protons that have passed through the polymer electrolyte membrane and the electrons that have passed through the external load circuit to form water.
A key feature of PEMFCs, therefore, is the nature of the polymer electrolyte membrane (PEM) interposed between the anode and the cathode. Often this membrane is referred to as a proton exchange membrane (also PEM) given the requirement of the membrane to facilitate the migration of protons (but not electrons) within the fuel cell. In addition to these functions, the membrane must not permit the passage of gas in either direction and be able to withstand the reductive and oxidative chemistries taking place at the cathode and anode respectively.
The polymer electrolyte Nafion®, which is a sulfonated tetrafluroroethylene-based fluoropolymer-copolymer discovered in the 1960's, is probably the PEM most commonly used. The utility of Nafion® in PEMFCs is believed to arise from its ability to transport protons as a consequence of its pendant sulfonic acid side groups, but that it is electrically insulating to anions or electrons. Over time, Nafion® loses fluorine from its structure. Nafion® relies on the presence of water to function as a conductor of protons. This means that PEMFCs employing Nafion® as PEM are restricted to operating temperatures of less than 100° C., implying low-temperature applications. At temperature approaching and in excess of 100° C., so-called fuel cell dehydration takes place the PEM becomes too dry to conduct protons to the cathode effectively resulting in a drop in power output. This illustrates a particular difficulty inherent to PEMFCs: the presence and maintenance of appropriate amounts of water. Effective management of the water generated within PEMFCs is a key issue in relation to the success of PEMFCs. Whilst problems can exist in Nafion®-based PEMFCs, with other PEMs too much water can also be detrimental.
It would be advantageous to expand the range of potential application of PEMFCs, in particular to further their use in electric vehicles (EVs). Since automotive air cooling systems can operate effectively at temperatures of around 130 to 140° C., increasing the temperature at which PEMFCs can function would be particularly advantageous to the automotive industry as it seeks to accelerate research into the incorporation of PEMFCs into EVs on account of the present environmental and economic climate. Being able to operate PEMFCs at this temperature range would obviate the need for expensive cooling systems which are otherwise be necessary where PEMFCs employ PEMs such as Nafion®.
Accordingly, an increasingly popular approach taken with PEMFCs is to focus on high temperature PEMFCs—HTPEMFCs—in which alternative polymers such as polybenzimidazole (PBI) are used on account of their high thermal stability. Unfortunately, a disadvantage with PBI is observed in its pure state is a very low conductivity of the order 10−12 S/cm. Improved conductivities have been found when PBI is doped at relatively high levels of with phosphoric acid (typically 5 to 7 moles of H3PO4 per unit of monomer of PBI) resulting in PBI—H3PO4. PBI—H3PO4 has been reported by O E Kondsteim et al. (Energy 32 (2007) 418-422) to possess conductivity of approximately 6.8×10−2 S/cm at 200° C. with approximately 560 mol % pyrophosphoric acid (equating to about 5 molecules of H3PO4 per repeat unit within the PBI). However, a further disadvantage of PBI-based PEMs is the decrease in mechanical strength that takes place within increasing temperature and increased level of doping. Also, acid leaches out at temperatures of about 160° C.
A third PEM of potential use in HTPEMFCs is not based upon the use of a polymer but rather the use of heteropolyacids (HPAs), such H6P2W21O71, which has been reported to exhibit good conductivity, dependant on relative humidity K A Record et al., (US Department of Energy Journal of Undergraduate Research, VI (2006), 53-58); and L Wang, (Electrochimica Acta 52 (2007), 5479-5483)).
Polymer composites are mentioned in WO2007/082350 and are described as comprising at least one inorganic proton-conducting polymer functionalised with at least one ionisable group and/or at least one hybrid proton-conducting polymer functionalised with at least one ionisable group, and at least one organic polymer capable of forming hydrogen bonds.
There remains an ongoing need for the provision of proton exchange membranes suitable for use in (HT)PEMFCs which can operate at temperatures in excess of 100° C., and ideally, be less dependent upon the relative humidity within the (HT)PEMFC.